The present invention relates to a corrosion-resistant metallic material or member, a metallic separator for fuel cells which comprises the metallic member, and a process for producing them. More particularly, the invention relates to a highly corrosion-resistant material or member improved in corrosion resistance, adhesion, contact electrical resistance, electrical conductivity, airtightness, etc. and suitable for use as a metallic separator for polymer electrolyte fuel cells (PEFC), and to a process for producing the same.
In applications where corrosion resistance is required, highly corrosion-resistant materials such as, e.g., stainless steel, nickel-based alloys, titanium, and titanium alloys have hitherto been used as they are, or materials obtained by plating steel, stainless steel, or the like with copper, nickel, chromium, or the like have been used. In applications where higher corrosion resistance is required, materials obtained by plating stainless steel or another base with a noble metal such as, e.g., gold or platinum have generally been used.
In producing plated products, plating is conducted after the base has been formed into the shape of the final product, because forming after plating may result in peeling of the deposit. There has hence been a problem that a film is less apt to be deposited by plating at corners such as groove edges and the plated product thus obtained has poor corrosion resistance in these parts.
Deposit films formed by plating have a porous structure and hence have poor adhesion to the base. In addition, since deposit films have pinholes, they have reduced corrosion resistance when they are thin. Although increasing the deposit thickness is necessary for heightening corrosion resistance, this poses a problem of increased cost in the case of noble-metal plating.
A metallic separator for polymer electrolyte fuel cells (PEFC) functions not only to electrically connect an electrode of a unit cell to an electrode of an adjacent unit cell but also to separate the reaction gas. The separator should therefore have high electrical conductivity and high gastightness, i.e., high impermeability to the reaction gas. Furthermore, the separator should have high corrosion resistance in the reactions in which hydrogen/oxygen is oxidized/reduced.
A metallic separator which has been known as a separator for polymer electrolyte fuel cells (PEFC) is one produced by a process comprising cutting a carbon plate such as a graphite plate to form therein many grooves arranged in corrugations for passing a fuel gas or oxidizing gas therethrough. This process, however, has a problem that the costs of the carbon plate material and the cutting are high, so that the separator produced by the process is too costly to be used practically.
Another metallic separator for polymer electrolyte fuel cells (PEFC) is disclosed in JP-A-10-228914. This separator is produced by pressing a stainless-steel plate to form therein many grooves arranged in corrugations for passing a fuel gas or oxidizing gas therethrough and then directly plating the edges of the protruding tips with gold in a thickness of from 0.01 to 0.02 xcexcm.
JP-A-2000-21418 discloses a metallic separator produced by pressing an SUS 316 plate to form therein many grooves arranged in corrugations for passing a fuel gas or oxidizing gas therethrough and then subjecting the surface thereof to nickel strike, nickel plating, and gold plating.
The separators for fuel cells which have been proposed further include one produced by forming a metal plate into a given shape, depositing a thin metal layer on at least one side thereof, and then filling up the pinholes in the thin metal layer by roller pressing, anodization, or resin coating (see JP-A-2001-68129).
However, those techniques of the related art have problems that the corrosion-resistant metallic member produced still has insufficient corrosion resistance, that in some of the techniques, there are limitations on the materials of metals usable as the base, and that the production of the corrosion-resistant metallic member is laborious.
Furthermore, the technique of the related art in which a metal film is deposited by plating on a surface having grooves formed therein beforehand has a drawback that there are cases where voids remain between the deposit film and the stainless steel and where the deposit film has too small a thickness at the edges (corners) of the groove tops. In addition, there have been problems that since the deposit film has a porous structure, it has poor adhesion to the stainless steel, and that the stainless steel corrodes through the pinholes and pores of the deposit film.
An object of the present invention is to provide a highly corrosion-resistant inexpensive material which has no limitations on the materials of metals usable as the base, is improved in corrosion resistance, adhesion, contact electrical resistance, and other properties, and is usable as, e.g., a metallic separator for polymer electrolyte fuel cells (PEFC). Another object of the invention is to provide a process for producing the material.
Still another object of the invention is to provide a corrosion-resistant metallic member having high corrosion resistance and low electrical resistance and suitable for mass production. Further objects of the invention are to provide a metallic separator for fuel cells which comprises the metallic member, and to provide a process for producing the separator.
In order to eliminate the problems described above, attention was directed to the coating of a surface of a metallic base made of any desired material with a thin noble-metal layer having a dense structure and retaining high adhesion strength.
Intensive investigations were made in order to develop a corrosion-resistant material or corrosion-resistant member which is improved in corrosion resistance and contact electric resistance and is inexpensive, a metallic separator for polymer electrolyte fuel cells (PEFC) which comprises the corrosion-resistant material or member, and a process for producing the same. As a result, it has been found that when a base coated with a film of a noble metal deposited by plating or another technique is rolled together with the coating film, then not only almost the same adhesion strength as in clad metals is obtained but also the porous structure of the coating film is densified and pinholes are filled up, whereby corrosion resistance is improved. It was also found that since adhesion strength is enhanced, the coating film does not peel off even when passages for passing a fuel gas or oxidizing gas are formed thereafter by plastic working. The following have been further found. Since corrosion resistance is improved, the thickness of the coating film can be reduced, leading to a cost reduction. Because of the surface coating layer made of a noble metal, the corrosion-resistant member has reduced contact electrical resistance. Furthermore, a preferred process for producing the corrosion-resistant member was found to comprise depositing a thin noble-metal layer on the desired part of the surface of a metallic base, compression-working the thin noble-metal layer, and then subjecting the coated base to an anticorrosive treatment with a liquid phase containing a peroxide or ozone or with an active gas atmosphere.
The present invention was accomplished based on these findings.
The invention provides a corrosion-resistant metallic member which comprises a metallic base and a thin noble-metal layer deposited on at least part of the metallic base and which has undergone compression working to reduce the total thickness of the base and the thin layer by 1% or more (preferably 5% or more).
The highly corrosion-resistant material (member) of the invention is preferably one which comprises a metallic material, e.g., an elemental metal selected from the group consisting of iron, nickel, titanium, copper, and aluminum or an alloy comprising at least one metal selected from said group, and deposited on a surface thereof a noble-metal layer made of, e.g., one metal selected from the group consisting of gold, platinum, palladium, silver, rhodium, and ruthenium, or an alloy comprising at least one metals selected from said group, and in which the base and the deposit layer has undergone compression working to reduce the total thickness of the base and the thin layer by 1% or more (preferably 5% or more).
The highly corrosion-resistant material (member) of the invention is more preferably one which comprises a metallic material, e.g., an elemental metal selected from the group consisting of iron, nickel, titanium, copper, and aluminum or an alloy comprising at least one metal selected from said group, and deposited on a surface thereof a noble-metal layer made of, e.g., one metal selected from the group consisting of gold, platinum, palladium, silver, rhodium, and ruthenium, or an alloy comprising at least one metals selected from said group, and in which the base and the deposit layer has undergone compression working to reduce the total thickness of the base and the thin layer by 1% or more (preferably 5% or more) and the work hardening resulting from the rolling has been removed by heating the clad metal under such conditions as not to diffuse and eliminate the noble-metal layer and as to be suitable for the base.
The invention further provides a process for producing a corrosion-resistant metallic member which comprises the steps of: depositing at least one noble metal on at least part of a metallic base to form a thin noble-metal layer; and compressing the resultant noble-metal-coated metallic material to reduce the total thickness of the base and the thin layer by 1% or more (preferably 5% or more).
The process of the invention for producing a highly corrosion-resistant material preferably comprises depositing one or more noble metals such as one metal selected from the group consisting of gold, platinum, palladium, silver, rhodium, and ruthenium, or an alloy comprising at least one metals selected from said group on a surface of a metallic material, e.g., an elemental metal selected from the group consisting of iron, nickel, titanium, copper, and aluminum or an alloy comprising at least one metal selected from said group, by plating or another technique and then compressing the base and the deposit layer at a draft of 1% or higher (preferably 5% or higher).
The process of the invention for producing a highly corrosion-resistant material more preferably comprises depositing one or more noble metals such as one metal selected from the group consisting of gold, platinum, palladium, silver, rhodium, and ruthenium, or an alloy comprising at least one metals selected from said group on a surface of a metallic material, e.g., an elemental metal selected from the group consisting of iron, nickel, titanium, copper, and aluminum or an alloy comprising at least one metal selected from said group, by plating or another technique, subsequently compressing the base and the deposit layer at a draft of 1% or higher (preferably 5% or higher), and then conducting a heat treatment in which the work hardening resulting from the rolling is removed under such conditions as not to diffuse and eliminate the noble-metal layer and as to be suitable for the base.
According to the highly corrosion-resistant material of the invention and the process of the invention for producing the same, a metallic material such as, e.g., an iron-based alloy and a coating film made of, e.g., one metal selected from the group consisting of gold, platinum, palladium, silver, rhodium, and ruthenium, or an alloy comprising at least one metals selected from said group, deposited on a surface thereof are rolled together to thereby clad the base. Because of this, not only almost the same adhesion strength as in clad metals is obtained, but also the porous structure of the noble-metal layer is densified and pinholes are filled up, whereby corrosion resistance is improved.
The improvement in corrosion resistance enables the thickness of the noble-metal layer, e.g., gold layer, to be reduced, leading to a cost reduction.
Due to the noble-metal layer, e.g., gold layer, deposited on the surface, the corrosion-resistant material has excellent corrosion resistance and reduced contact electrical resistance. In the case where the corrosion-resistant material has undergone a heat treatment, it further has excellent workability.
The invention furthermore provides a metallic separator for fuel cells which comprises the corrosion-resistant metallic member described above which has, on at least one of the front and back sides thereof, passages for enabling a fuel gas or oxidizing gas to flow therethrough.
The metallic separator for polymer electrolyte fuel cells (PEFC) of the invention is preferably one which comprises a metal plate made of, e.g., an elemental metal selected from the group consisting of iron, nickel, titanium, copper, and aluminum or an alloy comprising at least one metal selected from said group, and deposited on a surface thereof a noble-metal layer made of, e.g., one metal selected from the group consisting of gold, platinum, palladium, silver, rhodium, and ruthenium, or an alloy comprising at least one metals selected from said group, and in which the base and the deposit layer has undergone compression working to reduce the total thickness of the base and the thin layer by 1% or more (preferably 5% or more) and passages for passing a fuel gas or oxidizing gas therethrough have been formed by pressing or another technique.
The metallic separator for polymer electrolyte fuel cells (PEFC) of the invention is more preferably one which comprises a metal plate made of, e.g., an elemental metal selected from the group consisting of iron, nickel, titanium, copper, and aluminum or an alloy comprising at least one metal selected from said group, and deposited on a surface thereof a noble-metal layer made of, e.g., one metal selected from the group consisting of gold, platinum, palladium, silver, rhodium, and ruthenium, or an alloy comprising at least one metals selected from said group, and in which the base and the deposit layer has undergone compression working to reduce the total thickness of the base and the thin layer by 1% or more (preferably 5% or more), the work hardening in, e.g., the metal plate resulting from the rolling has been removed by heating the clad material under such conditions as not to diffuse and eliminate the coating film and as to be suitable for the base, and passages for passing a fuel gas or oxidizing gas therethrough have been formed by pressing or another technique.
A preferred process according to the invention for producing the metallic separator for polymer electrolyte fuel cells (PEFC) comprises depositing one or more noble metals such as one metal selected from the group consisting of gold, platinum, palladium, silver, rhodium, and ruthenium, or an alloy comprising at least one metals selected from said group on a surface of a metal plate made of, e.g., an elemental metal selected from the group consisting of iron, nickel, titanium, copper, and aluminum or an alloy comprising at least one metal selected from said group, subsequently compressing the base and the deposit layer at a draft of 1% or higher (preferably 5% or higher), and then subjecting the clad metal to working, e.g., pressing, to form passages for passing a fuel gas or oxidizing gas therethrough.
A more preferred process according to the invention for producing the metallic separator for polymer electrolyte fuel cells (PEFC) comprises depositing one or more noble metals such as one metal selected from the group consisting of gold, platinum, palladium, silver, rhodium, and ruthenium, or an alloy comprising at least one metals selected from said group on a surface of a metal plate made of, e.g., an elemental metal selected from the group consisting of iron, nickel, titanium, copper, and aluminum or an alloy comprising at least one metal selected from said group, compressing the base and the deposit layer at a draft of 1% or higher (preferably 5% or higher), subsequently conducting a heat treatment in which the work hardening in, e.g., the metal plate resulting from the rolling is removed under such conditions as not to diffuse or eliminate the coating film and as to be suitable for the base, and then subjecting the clad metal to working, e.g., pressing, to form passages for passing a fuel gas or oxidizing gas therethrough.
According to the metallic separator for polymer electrolyte fuel cells (PEFC) of the invention and the process for producing the same, a metal plate such as, e.g., a plate of an elemental metal selected from the group consisting of iron, nickel, titanium, copper, and aluminum or an alloy comprising at least one metal selected from said group and a coating film made of, e.g., one metal selected from the group consisting of gold, platinum, palladium, silver, rhodium, and ruthenium, or an alloy comprising at least one metals selected from said group, deposited on a surface thereof are rolled together to thereby clad the metal plate. Because of this, not only almost the same adhesion strength as in clad metals is obtained, but also the porous structure of the noble-metal layer is densified and pinholes are filled up, whereby corrosion resistance is improved.
The improvement in corrosion resistance enables the thickness of the noble-metal layer, e.g., gold layer, to be reduced, leading to a cost reduction.
Due to the noble-metal layer, e.g., gold layer, deposited on the surface, the metallic separator has excellent corrosion resistance and reduced contact electrical resistance. In the case where the metallic separator has undergone the heat treatment for removing work hardening, it further has excellent workability.
The corrosion-resistant metallic member (material) of the invention preferably comprises a metallic base and a thin noble-metal layer deposited on at least one of the front side and back side of the metallic base, wherein the adhesion strength between the metallic base and the thin noble-metal layer is preferably 50% or lower in terms of the amount of the layer peeling off in a peeling test after a corrosion test.
This metallic member can have high corrosion resistance and low contact resistance and can be easily mass-produced, because the thin noble-metal layer has been deposited on the front and/or back side of the metallic base at high adhesion strength. In case where the amount of coating layer peeling off exceeds 50%, the thin noble-metal layer has reduced adhesion strength and is apt to peel off and the metallic member hence deteriorates in corrosion resistance. Consequently, the amount thereof should be 50% or below, and is preferably 10% or below. The peeling test is conducted in accordance with JIS Z 0237. In this test, the areal proportion of the thin noble-metal layer peeled off with a tape is taken as the amount of coating layer peeling off. The corrosion test is conducted by holding the metallic member in a boiling sulfuric acid solution (atmosphere) having a pH of 2 for 168 hours. However, it is noted that this corrosion test is not intended to corrode the metallic base or noble metal.
The corrosion-resistant metallic member of the invention includes one in which the thin noble-metal layer has a thickness of from 0.1 to 100 nm and is constituted of a dense structure. In this metallic member, the thin noble-metal layer is less apt to peel off in the peeling test even when the metallic member has undergone the corrosion test. Consequently, corrosion resistance is maintained even when the noble-metal layer has a thickness smaller than in corrosion-resistant metallic members heretofore in use. Because of this, stable corrosion resistance can be obtained at low cost.
The reasons for the thickness range shown above are as follows. In case where the thickness of the thin noble-metal layer is smaller than 0.1 nm, the metallic member has reduced corrosion resistance and is unsuitable for practical use. On the other hand, thicknesses thereof exceeding 100 nm result in a cost increase. The preferred range of the thickness of the thin noble-metal layer is from 5 to 50 nm. The term xe2x80x9cdense structurexe2x80x9d used above means a metallic structure which is evenly adherent to the front/back side of the metallic base and is formed by compression-working the thin noble-metal layer which will be described later, such as, e.g., a noble-metal layer deposited by plating.
The corrosion-resistant metallic member of the invention further includes one in which the thin noble-metal layer deposited on at least one of the front and back sides of the metallic base has undergone compression working to reduce the total thickness of both (the metallic base and the thin noble-metal layer) by 1% or more (preferably 5% or more). According to this constitution, a corrosion-resistant member in which the thin noble-metal layer has been deposited on the front/back side of the metallic base at high adhesion strength so as to have a dense structure free from pinholes, pores, or the like can be provided without fail.
In case where the degree of compression in the compression working is lower than 1%, the thin noble-metal layer has insufficient adhesion strength. Consequently, the degree of compression should be 1% or higher, preferably 5% or more, and more preferably 10% or higher, most preferably 30% or higher. Examples of the compression working include rolling and pressing. In the rolling, the degree of compression is referred to as draft.
The corrosion-resistant metallic member of the invention still further includes one in which the thin noble-metal layer has been formed by depositing one metal selected from the group consisting of gold, platinum, palladium, silver, rhodium, and ruthenium, or an alloy comprising at least one metals selected from said group, on the metallic base by plating, screen printing, PVD, or CVD. According to this constitution, the thin noble-metal layer can be precisely deposited on the front/back side of the metallic base in a thickness of from 0.1 to 100 nm.
Examples of the PVD include vapor deposition, sputtering, and ion plating.
The corrosion-resistant metallic member of the invention furthermore includes one in which the metallic base comprises an elemental metal selected from the group consisting of iron, nickel, titanium, copper, and aluminum or an alloy comprising at least one metal selected from said group. According to this constitution, a corrosion-resistant metallic member is obtained which comprises a metallic base made of any of these materials and a thin noble-metal layer which has an extremely small thickness such as that shown above and has been deposited on the base at high adhesion strength so as to have a dense structure. Thus, the corrosion-resistant metallic member can be provided at an optimal cost according to various applications where high corrosion resistance and low electrical resistance are required.
On the other hand, the metallic separator for fuel cells of the invention preferably is one which comprises a metallic base and a thin noble-metal layer deposited on at least one of the front and back sides of the metallic base, and in which the adhesion strength between the metallic base and the thin noble-metal layer is 50% or lower in terms of the amount of the layer peeling off in a peeling test after a corrosion test.
In this constitution, the thin noble-metal layer has been deposited on the front/back side of the metallic base at high adhesion strength. Consequently, this metallic separator for fuel cells combines high corrosion resistance and low contact electrical resistance and is suitable for mass production.
The metallic separator for fuel cells of the invention includes one in which the thin noble-metal layer has a thickness of from 0.1 to 100 nm and is constituted of a dense structure. In this constitution, the noble-metal layer has a smaller thickness and a denser structure than in metallic separators heretofore in use. Consequently, a metallic separator for fuel cells which has high corrosion resistance and low contact resistance can be provided at low cost.
The metallic separator for fuel cells of the invention further includes one in which at least one of the front and back sides of the metallic base is coated with the thin noble-metal layer and has passages for enabling a fuel gas or oxidizing gas to flow therethrough.
According to this constitution, the highly corrosion-resistant thin noble-metal layer has been deposited at high adhesion strength over the front/back side having the passages, and the separator has low contact resistance. Consequently, this separator can have excellent dimensional accuracy with respect to the surface shape of the passage-bearing side which has a corrugated section, and is hence suitable for practical use.
The metallic separator for fuel cells of the invention still further includes one in which the metallic base and the thin noble-metal layer deposited on at least one of the front and back sides thereof have undergone compression working to reduce the total thickness of both (the metallic base and the thin noble-metal layer) by 1% or more (preferably 5% or more). In this separator, the thin noble-metal layer has been deposited on the front/back side of the metallic base so as to have a dense structure which attains high adhesion strength and high corrosion resistance.
The metallic separator for fuel cells of the invention furthermore includes one in which the thin noble-metal layer has been formed by depositing one metal selected from the group consisting of gold, platinum, palladium, silver, rhodium, and ruthenium, or an alloy comprising at least one metals selected from said group, by plating, screen printing, PVD, or CVD. This separator can be one in which the thin noble-metal layer has been precisely deposited on the front/back side of the metallic base in a thickness of from 0.1 to 100 nm.
The metallic separator for fuel cells of the invention furthermore includes one in which the metallic base comprises an elemental metal selected from the group consisting of iron, nickel, titanium, copper, and aluminum or an alloy comprising at least one metal selected from said group. This separator can be one which is relatively inexpensive and comprises a metallic base made of any of these materials and a thin noble-metal layer which has an extremely small thickness such as that shown above and has been deposited on the front/back side of the base at high adhesion strength so as to have a dense structure.
In each of the embodiments of the invention described above, it is preferred to further conduct an anticorrosive treatment with a liquid phase containing a peroxide or ozone or with an active gas atmosphere.